In the cultivation of pot plants such as bonsai, water management can be listed as the most important and difficult problem. That is, overwatering may rot the roots, while underwatering may kill the plants. In a conventional water management method for pot plants, for example, the following procedures are repeatedly carried out: a prescribed amount of water is supplied periodically everyday, or; the degree of wetness of the soil in which the pot plants were planted is observed periodically, and when the soil looked dry water is supplied to sufficiently cancel the dryness.
However, for common soil, the water content becomes very high and is likely to be excessive immediately after watering, and on the other hand, later the water evaporates quickly and the water content drops and is likely to cause a water shortage. Also, with the methods such as those described above, watering has to be done daily or every few days. This not only requires a lot of time and work but also means that the plants may die if watering stops for a long period of time for reasons such as when the caretaker is away from the home. Supplying an excessive amount of water, i.e. more than usual in order to prevent that, can cause root rotting.